The invention relates to an improved method and apparatus for generating the singlet delta electronically excited state of diatomic oxygen, O2(a1Δg), in vapor form. The apparatus for generating singlet delta oxygen is referred to as a singlet oxygen generator or SOG. Singlet delta oxygen is most typically used in chemical lasers, or specifically in chemical oxygen-iodine lasers (COIL), although there may be other uses for singlet delta oxygen and particularly for singlet delta oxygen generated according to the method and using the SOG of the invention.
Singlet delta oxygen is generally produced by reacting aqueous basic hydrogen peroxide (BHP) with chlorine. Aqueous BHP is produced by mixing liquid water with an aqueous solution of hydrogen peroxide (H2O2) and an aqueous solution of potassium hydroxide (KOH). Alternatively, sodium hydroxide (NaOH) may be used in place of KOH. In the BHP solution, the H2O2 and KOH exist as various ionic molecules. The mixing and reactive process in making BHP is exothermic.
When BHP is mixed with chlorine, the following stoichiometric chemical reaction takes place:H2O2+2KOH+Cl2→2KCl+2H2O+O2  Reaction Awhere the oxygen is in its lowest energy electronically excited state, O2(a1Δg). For convenience, this is referred to as singlet delta oxygen or as O2(1Δ). Normally, oxygen is in its electronic ground state, O2(X3Σg−), which, hereafter, is written as O2(3Σ) or just O2. In Reaction A, the chlorine vapor diffuses into the aqueous BHP solution, forming potassium chloride (KCl), or sodium chloride (NaCl) if NaOH is used in the reaction, water, and O2(1Δ). The O2(1Δ) can form bubbles and diffuse out of the solution. The presence of singlet delta oxygen from the reaction of BHP and chlorine in Reaction A is evident by a red dimol emission (see “Direct Spectroscopic Evidence for a Deuterium Solvent Effect on the Lifetime of Singlet Delta Oxygen in Water,” Kajiwara and Kearns, Journal of the American Chemical Society, vol. 95, No. 18, pp. 5886–5890, September 1973) that is visible by sight. This emission stems from the chemiluminescence of (O2(a1Δ))2.
Singlet delta oxygen has a long radiative lifetime of about 90 minutes, but can collisionally deactivate in much less time, resulting in the production of the ground state and the next higher electronically excited state, O2(1Σ), of diatomic oxygen. An important process is the gas-phase pooling reaction:O2(1Δ)+O2(1Δ)←→O2(3Σ)+O2(1Σ)  Reaction Bwherein O2(1Σ) is shorthand for O2(b1Σg+), which is a more energetic electronic state than singlet delta oxygen. The O2(1Σ) and O2(3Σ) are contaminants or byproducts in the singlet delta oxygen stream, which reduce the yield of singlet delta oxygen. When O2(1Δ) is the preferred product, as is the case with the chemical laser, then Reaction B is a deactivation process to the extent that the forward rate exceeds the backward rate.
Reaction B is a gas-phase process that can be viewed as producing the O2(1Σ) state. In non-laser applications, this state may be preferred as equal to or superior to the O2(1Δ) state. Hereafter, when discussing the singlet delta state of oxygen, as generated by an SOG according to the invention, the O2(1Σ) state, produced by Reaction B, is not excluded.
An important deactivation process is the dimol reaction:O2(1Δ)+O2(1Δ)→(O2(a1Δ))2  Reaction CReactions B and C are the primary deactivation process for removing O2(1Δ) in the gas phase. The gas phase generated by the SOG of this invention consists of O2(1Δ) (which is the preferred species for COIL), O2(3Σ), O2(1Σ), H2O, and possibly a small mole fraction of chlorine vapor. The O2(3Σ), O2(1Σ), H2O and chlorine vapor (if any) are referred to as the gas-phase byproducts. In addition, there may or may not be added diluent, which is not a byproduct. For COIL, a common measure of effectiveness is the yield, which is the mole ratio of O2(1Δ) divided by the total oxygen.
Various types of singlet delta oxygen generators have been developed in the prior art. These generators typically use BHP with chlorine and a diluent gas, such as helium. Optimum singlet delta oxygen production occurs when the H2O2 molar flow rate, relative to the KOH, or NaOH, molar flow rate is about double that suggested by Reaction A; i.e., H2O2 and KOH have approximately the same molar flow rate. These molar flow rates were used in the feasibility experiment according to the invention, as described later.
One type of prior art SOG uses a transverse flow uniform droplet method in which BHP droplets, ranging in size from 0.4 mm to 0.5 mm (15.8 mil to 19.7 mil) diameter, fall under the influence of gravity into a sump. Chlorine vapor and a diluent gas flow across the path of the droplets. The flow speed of the chlorine vapor and diluent is limited, otherwise the droplets would be transported downstream with the diluent and the generated oxygen. There is an adverse trade-off in that the maximum vapor speed, which includes the generated singlet delta oxygen, must decrease as the droplet size decreases. Generator pressures of around 92 Torr (0.12 atm), most of which is due to helium diluent, have been reported in this type of SOG. The partial pressure of the generated oxygen reported for this type of SOG is only around 14.3 Torr (0.02 atm).
Another type of prior art SOG is a verticoil oxygen generator. In this device, a number of disks rotate such that the lower portion of the disks is in a BHP sump. The upper portion of the disks is thus wetted with a BHP film. Chlorine vapor and diluent flow past the upper part of the disks to react with the BHP film. Generator pressures of about 40 Torr (0.05 atm), most of which stems from the helium diluent that enters the reactor with the chlorine vapor, have been reported in this type of SOG.
Another type of prior art SOG is a twisted-flow aersol-jet singlet oxygen generator. A partial pressure of about 75 Torr (0.1 atm) of singlet delta oxygen has been reported for this type of generator, but this O2(1Δ) pressure decreases, to around 22.5 Torr (0.03 atm), at a nozzle inlet for a laser. This significantly decreases the laser efficiency.
The foregoing prior art SOGs have, in common, a number of adverse characteristics:                (a) The devices are bulky, and typically require ducting to transport the O2(1Δ) stream to the nozzle that feeds the laser cavity. These devices are not well suited for scaling a laser module to a high-power level.        (b) Primarily, because of Reactions B and C, the singlet delta oxygen partial pressure entering the inlet of the laser's nozzle has not exceeded about 22.5 Torr (0.03 atm) in prior art systems. To increase the plenum pressure of the laser's nozzle, a diluent gas is used, typically helium or nitrogen. The need for supply tanks, plumbing, etc., to accommodate the use of a diluent gas further increases the size and weight of the overall system.        (c) The devices have difficulty keeping water vapor and water droplets from being entrained in the singlet delta oxygen stream. Water vapor is a deactivator that reduces the laser's performance. In many prior art systems, a water vapor trap and liquid separator, located between the SOG and laser nozzle, are needed. These traps, however, reduce the singlet delta oxygen concentration that enters into the laser's nozzle.        (d) Only a small percentage of the reactive chemicals in the BHP solution are utilized as the BHP flows through the oxygen generator. This results in a large and heavy BHP feed system, or a large and heavy system to recondition or regenerate the partly spent BHP.        (e) As described in “Mixed Marks for the ABL,” by Canan in Aerospace America, pp. 38–43, August 1999, Earth's gravitational field is required to provide buoyancy for separating the oxygen vapor from the liquid. Prior art SOG devices that rely on gravity for separation of singlet delta oxygen from the reactant flow, or that rely on gravity for reactant flow, such as the BHP droplets which fall by force of gravity in a transverse flow uniform droplet SOG, are not suitable for operation in a space environment.        (f) Prior art systems do not appear to yield a chemical efficiency for the laser above about 30%.        
In addition to prior art SOGs, gas sparger devices are relevant to the production of singlet delta oxygen according to the invention and as discussed more fully below. Gas sparger devices are designed to remove volatile contaminants from a liquid. In a prior art gas sparger, a contaminated liquid is injected, under pressure, onto the inside surface of a porous tube with a circular cross section. Centrifugal force keeps the liquid attached to the inside of the porous walled tube. The liquid follows a helical path as it makes a number of revolutions along the wall. Air, under pressure, is injected from the outside surface of the porous walled tube, through the tube, after which it mixes with the liquid. Most of the volatile contaminants are entrained with the air, which separates from the liquid, due to buoyancy that stems from the centrifugal force. Chemical reactions do not occur in typical gas spargers.